1. Field of the Invention
The present invention relates to a communication apparatus having the V. 8 procedure function for specifying a type of modem used in communication in line connection with a partner.
2. Related Background Art
In recent years, various types of communications such as computer communications and facsimile communications through a public telephone line have been performed. Modulators/demodulators called modems are used in these communications. In this case, no communications are performed unless the type of modem in a transmission apparatus is the same as that in a reception apparatus. The transmission rate of a modem and the modulation scheme have been set using the facsimile communication procedures, and computer communications using modems have been popular. For this reason, demand has arisen for specifying the types of modems of the transmitter and receiver.
Under the above circumstances, the V. 8 procedure is newly recommended from ITU as a procedure for specifying a modem used in communication.
When this V. 8 procedure is used for a facsimile apparatus having an image communication function using the conventional T. 30 procedure and an image communication function (V. 34 procedure) using a 28.8-kbps high-speed modem, the following sequence can be proposed.
The receiving side (called subscriber side) is connected to the line in response to a reception and starts sending out an ANSam signal (a 2,100-Hz modified response tone signal representing the V. 8 recommendation) 0.2 sec after the line connection. The receiving side receives a CM signal (originating menu signal that represents an available modulation scheme) from the transmitting side (calling subscriber side) while sending out the ANSam signal.
The transmitting side monitors the presence/absence of reception of the ANSam signal from the receiving side within a predetermined period of time after origination. When the ANSam signal is received within the above predetermined period of time, the transmitting side transmits the CM signal. On the other hand, when the transmitting side does not receive the ANSam signal within the predetermined period of time, the transmitting side transmits a CI signal (originating representation signal) representing the V. 8 procedure.
When the receiving side receives the CM signal within a predetermined period of time (5.+-.1 sec complying with the recommendation) from the start of sending out the ANSam signal, the receiving side stops transmitting the ANSam signal and transmits a JM signal (a common menu signal which represents an available modulation scheme common to the originating and reception sides).
Upon reception of the JM signal, the transmitting side transmits a CJ signal (a signal representing confirmation of detection of the JM signal and the end of the CM signal). In response to this, the transmitting side ends the V. 8 procedure sequence and advances to a data communication sequence using a modem in which the originating function category of the transmitted CM signal coincides with that of the received JM signal.
The receiving side stops sending out the ANSam signal when it does not receive the CM signal within the predetermined period of time. The receiving side then monitors reception of the CI signal from the transmitting side. Upon reception of the CI signal, the receiving side sends out the ANSam signal again. When the receiving side receives no CI signal, the receiving side transmits a DIS signal of the T. 30 procedure (i.e., this signal contains information having the V. 8 procedure function) and waits until the CI signal of the V. 8 procedure or the DCS signal of the T. 30 procedure is received from the transmitting side. Upon reception of the CI signal, the receiving side sends out the ANSam signal. Upon reception of the DCS signal, the receiving side performs an image communication sequence according to the T. 30 procedure.
In the above procedure, when the transmitting side is a normal facsimile apparatus of the T. 30 procedure, the receiving side sends out the ANSam signal for about 5 sec, and transmits the DIS signal after it confirms that neither the CM signal nor the CI signal are received from the transmitting side. Therefore, the T. 30 procedure is started.
Most of the facsimile communications are performed by the normal facsimile apparatus of the T. 30 procedure.
Even if the V. 8 procedure function is provided, the operation preferably returns to the T. 30 procedure within a minimum time period.
On the other hand, the sufficient detection time of the ANSam signal at the transmitting side is about one sec. The apparatus at the receiving side can receive a CM signal after about 2.2 sec in a line having a large communication delay between the terminals (e.g., satellite communication using a satellite).
The sending time of the ANSam signal at the receiving side is set to about 3 sec, so that the shift time to the T. 30 procedure can be advanced.
However, when the receiving side (called subscriber side) acquires the line in response to detection of a call signal from the line, a switching unit of a network inverts the line polarity. In the network of this switching unit, when the receiving side acquires the line, the line polarity is inverted. Noise is generated on the line by this polarity inversion, and the transmitting side may not normally receive the ANSam signal at the transmitting side upon polarity inversion.
For this reason, when the sending time of the ANSam signal is set to about 3 sec, and the switching unit of the network is of a polarity inversion type, the receiving side cannot receive the CM signal within 3 sec. As a result, after the CI signal is transmitted, the ANSam signal must be transmitted again, thereby prolonging execution of the V. 8 procedure.